The present invention relates to a power steering apparatus for reducing the force necessary to operate a steering apparatus for reducing the force necessary to operate a steeling wheel, and more particularly to a power steering apparatus which provides a suitable steering performance responsive to various car speeds and steering angles.
A power steering apparatus of the type to which the invention relates includes, as shown in FIG. 1, an oil pump 2 which pumps hydraulic fluid out of an oil tank (reservoir) 1, develops a fluid pressure, and supplies the pressurized oil to a power steering body; a control valve 3 which controls the flow of the pressurized oil supplied from the oil pump 2 according to the steering operation; and a power cylinder 4 having a piston 5 selectively shiftable in the axial direction according to which the pressurized oil is caused to flow into either a left chamber 4A or a right chamber 4B. Such movement of the piston 5 in the power cylinder 4 due to the flow direction of the pressurized oil is transmitted to a steering linkage mechanism (not shown) to facilitate the steering operation. Further, in such a power steering apparatus, hydraulic circuits in the power cylinder 4 may be changed by rotary movement of a steering wheel. Conventionally, a rotary valve having a rotor and sleeve has been used as the control valve 3 so as to change the flow channels of the pressurized oil.
In the conventional power steering apparatus as described above, the oil pump 2 serving to develop fluid pressure is normally rotated by the engine of a car. The flow rate of the pressurized oil supplied from the oil pump 2 is thus varied in proportion to the rate of rotation of the engine. Also, the capacity of the oil pump 2 is established in such a manner that an appropriate amount of pressurized oil may be supplied from the oil pump 2 so as to reduce the force necessary to turn the steering wheel when the car is stopped or travels at low speeds. In this manner, when the engine rotates at normal or high speeds, the amount of the pressurized oil discharged from the oil pump 2 tends to be greater than that which needs be supplied to the power steering body. It is for this reason that, conventionally, a flow control valve operating in response to the differential pressure between an orifice and its adjacent area is provided in the pressurized oil feed line. With this arrangement, excess amounts of the pressurized oil are returned to the oil tank 1 so that the pressurized oil may be supplied in a constant manner. It is also known to provide a flow control valve operable by variable orifice for reducing the flow rate of the pressurized oil when the car travels at high speeds. With this arrangement, a greater steering force is required at high speeds than at low speeds. Thus, the driver is provided with the proper "feel" on the steering wheel at any given speed.
Furthermore, it is well known to provide a speed-sensitive power steering system which includes a flow control section 7 having a variable orifice 6 adapted to vary the opening area in the feed line by moving a valve rod toward or away from the orifice by means of a solenoid coil 6a, as shown in FIG. 1. With this arrangement, opening and closing of the variable orifice 6 may be controlled by a controller 9, which receives signals from a car speed sensor 8. Upon such control, the flow rate of the pressurized oil at high speeds may appropriately be reduced. Accordingly, the steering "feel" is made desirably heavier at high speeds.
However, in such a prior art arrangement as above, only flow control is effected by the variable orifice 6. Accordingly, only slight changes in the flow rate of the pressurized oil in the feed line take place upon throttling the variable orifice 6 and the amount of variation in input torque (steering force) is therefore small. Namely, the prior art arrangement provides such characteristics as shown in FIG. 2A, representing the relationship between the state of opening and the opening area of the orifice, which is in proportion to the steering angle. Also, FIG. 2B shows the input torque in relation to the rack thrust (steering force plus auxiliary steering force by the present power steering apparatus). As is clear from FIGS. 2A and 2B, the difference between the flow rate of the pressurized oil when the variable orifice 6 is opened and that when the same is closed is small. Further, the relief pressure is constant and its variation with respect to steering force is small.
In order to appropriately control the steering force respective to car speed, there have previously been proposed arrangements such as disclosed in Japanese Laid-Open Patent Application No. 52-112922 and Laid-Open Utility Model Application No. 56-160174. However, these arrangements are complicated and require a large number of parts, and thus are costly. Furthermore, they are unable to provide suitable steering performance by establishing wide variations in the steering force. Hence, improved arrangements need be provided.
That is, in the former case, a by-pass line opened and closed by a valve provided in the feed line which extends from the oil pump to the power steering body for purposes of improving steering properties. However, this arrangement is complicated in structure. In addition, the flow of the pressurized oil tends to increase as the pressure in the feed line increases, and thus flow control may not adequately be effected. It is for this reason that a pressure control valve arrangement need be provided.
Further, in the latter case, there is provided a by-pass valve which communicates the left cylinder chamber with the right cylinder chamber in the power cylinder. However, this arrangement is complicated in structure and requires a large number of parts, and thus is costly. Therefore, there is a need for an improved arrangement which is simple in structure and which effects flow control responsive to speed and provides wide variations in the steering force.